For many years, a number of medical devices (e.g., pacemakers, vascular grafts, stents, heart valves, etc.) that contact bodily tissue or fluids of living persons or animals have been developed, manufactured and used clinically. A major problem with such articles is that their surfaces tend to adsorb a layer of proteins from tissues and fluids such as tears, urine, lymph fluid, blood, blood products, and other fluids and solids derived from blood. The composition and organization of this adsorbed protein layer is thought to influence, if not control, further biological reactions. Adverse biological reactions such as thrombosis and inflammation may diminish the useful lifetime of many devices.
Implantable medical devices may serve as foci for infection of the body by a number of bacterial species. Such device-associated infections are promoted by the tendency of these organisms to adhere to and colonize the surface of the device. Consequently, it has been of great interest to physicians and the medical industry to develop surfaces that are less prone to promote the adverse biological reactions that typically accompany the implantation of a medical device.
One approach for minimizing undesirable biological reactions associated with medical devices is to attach various biomolecules to their surfaces. Biomolecules such as antithrombogenics, antiplatelets, anti-inflammatories, antimicrobials, growth factors, proteins, peptides, and the like have been used to minimize adverse biomaterial-associated reactions.
A number of approaches have been suggested to attach such biomolecules. These approaches generally are covalent attachment techniques or ionic attachment techniques. Covalent attachment techniques typically require the use of coupling agents such as glutaraldehyde, cyanogen bromide, p-benzoquinone, succinic anhydrides, carbodiimides, diisocyanates, ethyl chloroformate, dipyridyl disulphide, epichlorohydrin, azides, among others, which serve as attachment vehicles for coupling of biomolecules to biomaterial surfaces. For example, covalent attachment of biomolecules using water soluble carbodiimides is described by Hoffman et al., "Covalent Binding of Biomolecules to Radiation-Grafted Hydrogels on Inert Polymer Surfaces," Trans. Am. Soc. Artif. Intern. Organs, 18, 10-18 (1972); and Ito et al., "Materials for Enhancing Cell Adhesion by Immobilization of Cell-Adhesive Peptide," J. of Biomed. Mat. Res., 25, 1325-1337 (1991).
Coupling molecules used for covalently attaching biomolecules to surfaces may create undesirable crosslinks between biomolecules, thereby destroying the biological properties of the biomolecule, or they may create bonds amongst surface functional sites, thereby inhibiting attachment. Covalent coupling of a biomolecule to a surface may also destroy the biomolecule's three-dimensional structure, thereby reducing or destroying the biological properties of the attached biomolecule, by altering its chemical composition.
Ionic coupling techniques have an advantage of not altering the chemical composition of the attached biomolecule, thereby reducing the possibility of destroying the biological properties of the attached biomolecule. Ionic coupling of biomolecules also has an advantage of releasing the biomolecule under appropriate conditions. One example of the ionic attachment of a biomolecule to a surface is set forth in U.S. Pat. No. 4,442,133 to Greco et al. In this case, a tridodecyl methylammonium chloride (TDMAC) coating is used to ionically bind an antibiotic agent.
Another type of biomolecule which is often coupled to biomaterial surfaces is heparin. Heparin, an anionic bioactive agent, is of great interest to a number of investigators for the development of non-thrombogenic blood-contact biomaterial surfaces. Heparin, a negatively charged glycosaminoglycan, inhibits blood coagulation primarily by promoting the activity of antithrombin III (ATIII) to block the coagulation enzymes thrombin, factor Xa and, to some extent, factors IXa, XIa and XIIa. Surfaces bearing bound heparin have been shown to have anticoagulant activity, therefore, heparinization tends to be a popular technique for improving the thromboresistance of biomaterials. In fact, surface heparinization through an ionic bond is one of the methods used to improve the blood comparability of a variety of biomaterial surfaces.
The original method of heparinization of surfaces was described by Gott et al., "Heparin Binding On Colloidal Graphite Surfaces", Science, 142, 1297-1298 (1963). They prepared a graphite-benzalkonium-heparin surface and observed good non-thrombogenic characteristics. Others followed, treating materials with quaternary ammonium salts to ionically bind heparin. Improving on Gott's technique, Grode et al., "Nonthrombogenic Materials via a Simple Coating Process", Trans. Amer. Soc. Artif. Intern. Organs, 15, 1-6 (1969), eliminated the need for a graphite coating by using tridodecyl methylammonium chloride (TDMAC). Various other quaternary ammonium salts have also been used such as benzalkonium chloride, cetylpyrrdinium chloride, benzyldimethylstearyammonium chloride, benzylcetyidimethylammonium chloride as set forth in U.S. Pat. No. 5,069,899 to Whitbourne and Mangan.
Glutaraldehyde was even used by some investigators to increase the stability of heparin bound ionically through various ammonium groups. Rather than using a low molecular weight quaternary salt or quaternary amine, many investigators incorporated the quaternizable amine directly onto substrates by copolymerization techniques. In another approach, Barbucci et al., "Surface-Grafted Heparinizable Materials", Polymer, 26, 1349-1352 (1985), grafted tertiary amino polymers of poly(amido-amine) structure onto substrates for ionically coupling heparin. The cationic amino groups are capable of interacting electrostatically with the negatively charged groups present in the heparin molecule. They found that the surface's capacity to retain heparin was directly related to the basicity of the grafted cationic amino groups. The greater the basicity of the surface amino groups on the surface, the greater the capacity of the surface has to retain heparin due to a greater percentage of the surface amino groups being protonated at physiological pH.
Current techniques for immobilization of heparin or other biomolecules by an ionic bond have been achieved by introducing positive charges on the biomaterial surface utilizing quaternary ammonium salts and polymers containing tertiary and quaternary amine groups. The main limit to long-term utilization of medical devices containing biomolecules immobilized by the above methods is the rapid release of the biomolecule with time. Therefore, a technique which may ionically bind anionic biomolecules to surfaces for greater lengths of time is highly desired.